[{"id":3679636,"web_url":"http://patchwork.ozlabs.org/comment/3679636/","msgid":"<986cbaf3-8958-e0dd-4e3b-9fe0425a730f@idea>","list_archive_url":null,"date":"2026-04-21T00:25:53","subject":"Re: [PATCH RFC] c++: constexpr union with no active member [PR124910]","submitter":{"id":78319,"url":"http://patchwork.ozlabs.org/api/people/78319/","name":"Patrick Palka","email":"ppalka@redhat.com"},"content":"On Mon, 20 Apr 2026, Jason Merrill wrote:\n\n> Tested x86_64-pc-linux-gnu. Since this isn't a regression I'm inclined to\n> leave it for 17.\n> \n> -- 8< --\n> \n> Patrick pointed out that while r16-8767 made a union constant after\n> destroying its active member, we still weren't treating a union that never\n> had an active member as constant; the difference is the\n> CONSTRUCTOR_NO_CLEARING flag, and what that means to\n> reduced_constant_expression_p.\n> \n> It seems to me that since P2686 [expr.const] says whether a prvalue\n> expression is a constant expression depends on the constituent values, and\n> [intro.object] says that only the active member is a constituent value of a\n> union, so a union with no active member has no constituent values and so is\n> vacuously constant, like an object of empty type. P2686 as a whole is not a\n> DR, but the draft was previously unclear, and CWG2658 also clarified that\n> copying a union is equivalent to copying the active member *if any*.\n> \n> I was somewhat surprised that none of the existing tests needed to be\n> changed.\n\nNice. FWIW that logic was added in r10-7313 (by me apparently) just so\nthat we reject cpp1y/constexpr-union4.C. Makes sense to leave it for\nGCC 17, I don't even think we have an existing PR about it.\n\n> \n> \tPR c++/124910\n> \tDR 2658\n> \n> gcc/cp/ChangeLog:\n> \n> \t* constexpr.cc (reduced_constant_expression_p): Allow a union\n> \twith no active member.\n> \n> gcc/testsuite/ChangeLog:\n> \n> \t* g++.dg/cpp2a/constexpr-union12.C: New test.\n> ---\n> gcc/cp/constexpr.cc | 7 ++++---\n> gcc/testsuite/g++.dg/cpp2a/constexpr-union12.C | 8 ++++++++\n> 2 files changed, 12 insertions(+), 3 deletions(-)\n> create mode 100644 gcc/testsuite/g++.dg/cpp2a/constexpr-union12.C\n> \n> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc\n> index a593b66dd41..1926b64cbb7 100644\n> --- a/gcc/cp/constexpr.cc\n> +++ b/gcc/cp/constexpr.cc\n> @@ -4747,9 +4747,10 @@ reduced_constant_expression_p (tree t, tree sz /* = NULL_TREE */)\n> \t\t && TREE_CODE (TREE_TYPE (t)) == UNION_TYPE)\n> \t {\n> \t if (CONSTRUCTOR_NELTS (t) == 0)\n> -\t\t/* An initialized union has a constructor element. */\n> -\t\treturn false;\n> -\t /* And it only initializes one member. */\n> +\t\t/* A union with no active member has no constituent values, so\n> +\t\t all constituent values are constant. */\n> +\t\tgoto ok;\n> +\t /* A union can have at most one active member. */\n> \t field = NULL_TREE;\n> \t }\n> \t else\n> diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-union12.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-union12.C\n> new file mode 100644\n> index 00000000000..42ddfaaf775\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-union12.C\n> @@ -0,0 +1,8 @@\n> +// PR c++/124910\n> +// { dg-do compile { target c++20 } }\n> +\n> +union U {\n> + int n;\n> + constexpr U() { }\n> +};\n> +constexpr U u; // { dg-bogus \"\" } U() is not a constant expression\n> \n> base-commit: 8e6b7c97232489557e376bdc134ad69aaeb35794\n> -- \n> 2.53.0\n> \n>","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=Pb5rJZUD;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; helo=vm01.sourceware.org;\n envelope-from=gcc-patches-bounces~incoming=patchwork.ozlabs.org@gcc.gnu.org;\n receiver=patchwork.ozlabs.org)","sourceware.org;\n\tdkim=pass (1024-bit key,\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=Pb5rJZUD","sourceware.org; dmarc=pass (p=quarantine dis=none)\n header.from=redhat.com","sourceware.org; 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